Fabric made of glass fiber is useful in filter applications to separate particles from a gas stream. Commonly, the glass fabric is coated with a tetrafluoroethylene polymer coating, usually polytetrafluoroethylene (PTFE), to increase the flex life, i.e., resistance to breaking on repeated flexing of the fabric. Such a coating is especially useful when the glass fabric is employed as a filter bag to filter out particulate solid impurities such as carbon black or fly ash which may be present in flue gases. Due to the passage of hot flue gases through the glass fabric and due to back flushing or pulse flushing during cleaning, the fabric is subjected to flexural stresses which weaken the glass fibers of the fabric and ultimately cause breakage.
While the tetrafluoroethylene polymer coating increases the flex life of the glass fabric, resistance of the coated glass fabric to attack by acids present in hot flue gases is of concern. Many hot flue gases contain oxides of sulfur and water vapor which combine to form an acidic environment. Glass fabric coated with tetrafluoroethylene polymer coatings heretofore were subject to attack by acids in the flue gas which weakened the fabric and shortened the fabric's flex life. Recently, U.S. Pat. No. 4,370,376 described a composition which, when coated on a glass fabric, imparted resistance to attack by acids. This composition was a tetrafluoroethylene aqueous dispersion which contained a selected silane, a selected siloxane and a fluorinated ester of acrylic acid. While these dispersions enhance flex life of glass fabrics on which the dispersions are coated, it is always desirable to increase flex life further.